Microglia in the rat neurohypophysis increase expression of class I major histocompatibility antigens following central nervous system injury

Abstract

An immunocytochemical study of the expression of major histocompatibility complex (MHC) class I and II antigens by glial cells of the rat neurohypophysis was performed. Numerous cells with the appearance of microglia were found to constitutively express class I MHC antigens, while only rare cells expressed class II (Ia) antigens. Stereological analysis revealed that expression of class I MHC antigens increased significantly within 10 days after a unilateral hypothalamic lesion known to cause axonal degeneration and compensatory collateral axonal sprouting within the neurohypophysis. In addition, however, a brain lesion which did not affect the axonal population of the neurohypophysis also produced a significant increase in microglial expression of class I MHC antigens in this structure. Neither lesion affected the expression of class II MHC antigens in the neurohypophysis. Simultaneous immunofluorescent labeling for MHC I antigens and glial fibrillary acidic protein (GFAP, a pituicyte marker) or for MHC I and the C3bi complement receptor (a microglial marker) confirmed that the MHC class I-reactive cells were microglia. MHC I-positive cells also bound Griffonia simplicifolia B4 isolectin (GSA I-B4), consistent with their identification as microglia. The majority of MHC class I-reactive microglia were located in close apposition to blood vessels. These results indicate that an unusually large proportion of microglia within the neurohypophysis constitutively express MHC I antigens. In addition, neurohypophysial microglia are capable of responding to penetrating brain injury by upregulation of MHC I antigens in the absence of local tissue degeneration, possibly because of the absence of a blood-brain barrier.